1. Field of the Invention
The present invention generally relates to a method of and an apparatus for detecting a vehicle speed pulse drop in an on-vehicle navigation system in which a current position etc. of a vehicle is displayed by using (i) a dead reckoning positioning system (i.e., a self-sustained or built-in positioning system), which is a system for measuring the current position on the basis of an output from dead reckoning positioning sensors including a vehicle speed sensor, and/or (ii) a radio wave positioning system such as a GPS (Global Positioning System), which is a system for positioning or measuring the current position on the basis of xe2x80x9cpositioning radio waves (which are radio waves or electric waves for positioning the present position)xe2x80x9d from satellites for the radio wave positioning or measurement such as GPS satellites etc. The present invention also relates to an on-vehicle navigation system including the above-mentioned detecting apparatus, as well as a program storage device and a computer data signal embodiment in a carrier wave, which allow a computer to function as the above-mentioned detecting apparatus.
2. Description of the Related Art
Currently, there are (i) the dead reckoning positioning or measuring system and (i) the GPS, as rough categories of a system for positioning or measuring the present position of a vehicle in an on-vehicle navigation system.
The dead reckoning positioning system is intended to calculate a current position with respect to a standard position, on the basis of a moving direction, a distance and the like of a movable body obtained by dead reckoning positioning sensors such as a vehicle speed sensor, an angular velocity sensor, an acceleration sensor and the like which are equipped in the movable body. Especially, the vehicle speed sensor, among the dead reckoning positioning sensors, is intended to generate a vehicle speed pulse in response to a rotation of a vehicle shaft after detecting the rotation and is constructed to calculate a drive distance and a velocity from the cumulative number and the number per unit time of the generated vehicle speed pulse respectively.
Recently, as disclosed in Japanese Patent Application Laying Open NO. Hei 10-300509 and Japanese Patent Application Laying Open NO. Hei 10-115627 etc., for example, there has been developed a vehicle speed sensor of a magnetic field detection type, which detects a change of a magnetic field emitted from a generation source of the vehicle speed pulse additionally attached to a tire or at the vicinity thereof, or emitted from a steel belt included in the tire, to thereby generate the vehicle speed pulse. According to this type of sensor, it is possible to generate the vehicle speed pulse relatively easily in an arbitrary type of vehicle including a foreign vehicle or the like which has no generation source of the vehicle speed pulse or is uncertain about its presence.
On the other hand, the GPS is intended to receive with GPS receivers the positioning radio waves from a plurality of GPS satellites launched into the outer space and to calculate a current position and a moving velocity of a movable body by a 3D (3-dimensional) measurement or a 2D (2-dimensional) measurement based on the reception results.
Recently, as disclosed in Japanese Patent Application Laying Open NO. Hei 09-297030 and Japanese Patent Application Laying Open NO. Hei 10-307036, for example, a navigation system, which performs both of the above described dead reckoning positioning measurement and the above described GPS measurement, has been also developed. This navigation system mainly uses the dead reckoning positioning measurement or compensates the GPS measurement with the dead reckoning positioning measurement in the case that the GPS measurement is not available or cannot attain an enough accuracy, for example. On the other hand, in the case that the GPS measurement is available or can attain an accuracy as usual, the navigation system mainly uses the GPS measurement and compensates the GPS measurement with the dead reckoning positioning measurement.
However, especially as to the vehicle speed sensor which plays a key role in the dead reckoning positioning system as described above, such a situation may occur that the xe2x80x9cvehicle speed pulsexe2x80x9d is not generated when xe2x80x9cthe vehicle speed pulsexe2x80x9d as a vehicle speed signal is supposed to be outputted from the vehicle speed sensor while the vehicle is actually driving,which is a problem (this situation is referred to as a xe2x80x9cvehicle speed pulse dropxe2x80x9d). There may be considered various causes and reasons of this vehicle speed pulse drop individually and concretely, e.g., it is difficult to produce (i) the vehicle speed sensor with a constant sensitivity and (ii) the generation source of the vehicle speed pulse with a constant output, over the whole vehicle speed, an electrical noise is generated on a line or wire through which the vehicle speed pulse is transmitted, and a sensor error is generated due to an unexpected vibration in driving.
Especially, as to the vehicle speed sensor which detects the change of the magnetic field to generate the vehicle speed pulse as mentioned above, there is a significant problem that the vehicle speed pulse drop may occur frequently depending on the driving condition because the magnetic field detected by the sensor may drastically change as much as it cannot be ignored on the detection accuracy depending on a strain of the tire, a suspension operation and so on.
When the vehicle speed pulse drop occurs, an error occurs such that the vehicle speed is drifted to its lower side to deteriorate the accuracy of the vehicle speed, and another error occurs such that the moving distance is drifted to its shorter side to deteriorate the accuracy of the moving distance based on the dead reckoning positioning system. Moreover, in addition to deteriorating the accuracy of the current position based on the dead reckoning positioning system, there is a problem that the vehicle speed pulse drop deteriorates the accuracy of the current position etc., which is finally obtained after the correction or compensation based on the dead reckoning positioning measurement with respect to the GPS measurement.
For this problem, Japanese Patent Application Laying Open NO. 2000-97713 discloses a system for calculating the moving distance and the current position with a high accuracy based on the vehicle speed pulse even in the case that the vehicle speed pulse drop is generated while driving at a low speed. However, a detection method of the vehicle speed pulse drop in this system is extremely simple. For example, in this system, the vehicle speed pulse drop is detected by a change from a condition of xe2x80x9cthe number of the vehicle speed pulse greater than 1xe2x80x9d to a condition of xe2x80x9cthe number of the vehicle speed pulse=0xe2x80x9d under monitoring the vehicle speed pulse with a constant time interval. Therefore, it is hard to say that the detection accuracy of the vehicle speed pulse drop is substantially high.
It is therefore an object of the present invention to provide a method of and an apparatus for detecting a vehicle speed pulse drop, in which the detection of the vehicle speed pulse drop can be performed with a high accuracy, an on-vehicle navigation system including the above mentioned detecting apparatus, as well as a program storage device and a computer data signal embodiment in a carrier wave, which allow a computer to function as the detecting apparatus.
The above object of the present invention can be achieved by a first method of detecting a vehicle speed pulse drop of a vehicle speed pulse in an on-vehicle navigation system, the system provided with a radio wave positioning apparatus and a vehicle speed sensor, by which the vehicle speed pulse is generated, for a dead reckoning positioning, the method provided with: a first calculation process of calculating a first physical quantity of a predetermined type from radio wave positioning data of the radio wave positioning apparatus; a second calculation process of calculating a second physical quantity of the predetermined type from the vehicle speed pulse; and a detection process of detecting the vehicle speed pulse drop by using a contradiction caused between the calculated first physical quantity and the calculated second physical quantity as a detection condition.
According to the first method of the present invention, the first calculation process calculates the first physical quantity of a predetermined type such as a velocity of a vehicle by using, for example, a frequency change caused by Doppler effect from the radio wave positioning data of the radio wave positioning apparatus such as a GPS receiver or the like. Along with this, the second calculation process calculates the second physical quantity of the same type as what is calculated in the first calculation process, e.g. a velocity, which is in proportion to the number of the vehicle speed pulse per unit time, of a vehicle, in which the on-vehicle navigation system is equipped, from the vehicle speed pulse such as the one from a vehicle speed sensor or the like. Then, the detection process detects the vehicle speed pulse drop by using a contradiction caused between the calculated first physical quantity and the calculated second physical quantity; for example, such a contradiction that there is an enormous difference between the calculated two kinds of velocities of the vehicle, as a detection condition. Namely, it is possible to detect the vehicle speed pulse drop when the velocity based on the vehicle speed pulse decreases so rapidly that it cannot be decelerated in a real world on the basis of vehicle weight, an ability of brake control or the like, or when the velocity based on the vehicle speed pulse decreases extremely as compared to the velocity based on the radio wave positioning data. Therefore, by using the radio wave positioning data with such a positioning accuracy that generally has no causal relationship with how easily the vehicle speed pulse drop occurs, it is possible to detect the vehicle speed pulse drop with a relatively high reliability. Thus, when the vehicle speed pulse drop occurs, it becomes possible to correct the vehicle speed pulse including an error of the vehicle speed sensor, or to eliminate a use of the vehicle speed pulse including the error for displaying a current position with the on-vehicle navigation system. For example, when the vehicle speed pulse drop is detected, it becomes possible to use the radio wave positioning data for navigation exclusively and temporarily, or to interpolate and predict a missing vehicle speed pulse by using the vehicle speed pulse in the past or the future when the vehicle speed pulse drop was not or will be not detected.
In one aspect of the first method, the first calculation process calculates a velocity as the first physical quantity, the second calculation process calculates a velocity as the second physical quantity, and the detection process uses such a condition that a difference between the velocity calculated by the first calculation process and the velocity calculated by the second calculation process is more than a predetermined threshold value as one detection condition for the vehicle speed pulse drop.
According to this aspect, the first calculation process calculates the velocity as the first physical quantity based on a positioning radio wave, and the second calculation process calculates the velocity as the second physical quantity based on the vehicle speed pulse, and the detection process detects the vehicle speed pulse drop by using such a condition that a difference between these velocities is more than the predetermined threshold value. For example, when the velocity calculated in the first calculation process is regarded as V1, the velocity calculated in the second calculation process as V2, and the threshold value as Vth, it is monitored whether or not a velocity difference V1xe2x88x92V2 greater than Vth. If this equation holds true, it is judged as the vehicle speed pulse drop. Therefore, it is possible to detect the vehicle speed pulse drop with a high reliability by a relatively simple process of monitoring inequality between the velocity difference and the threshold value.
Incidentally, in this aspect, it is possible to detect the vehicle speed pulse drop by comparing a velocity ratio V1/V2 with another threshold value Vthxe2x80x2 (i.e. a detection condition: V1/V2 greater than Vthxe2x80x2) in the detection process.
In this aspect of the first method, the method may be further provided with a third calculation process of calculating a velocity change amount from the radio wave positioning data and a fourth calculation process of calculating a velocity change amount from the vehicle speed pulse, wherein the detection process uses such a condition that the velocity change amount calculated by the third calculation process is less than a predetermined threshold value and the velocity change amount calculated by the fourth calculation process is more than a predetermined threshold value as another detection condition for the vehicle speed pulse drop.
By constituting in this manner, the vehicle speed pulse drop is detected by using such a condition that the velocity change amount calculated by the third calculation process is less than the predetermined threshold value and the velocity change amount calculated by the fourth calculation process is more than the predetermined threshold value as a detection condition in addition to the condition that the above mentioned velocity difference is more than the predetermined threshold value. In other words, the vehicle speed pulse drop is detected by using such a condition that, although the vehicle speed based on the radio wave positioning with a positioning accuracy, which generally has no causal relationship with how easily the vehicle speed pulse drop occurs, does not change substantially, the vehicle velocity based on the vehicle speed pulse changes relatively largely. In this manner, by using the fact that the velocity change of a vehicle is to some degree within a limited range, it is possible to detect the vehicle speed pulse drop with a very high accuracy, except extremely special cases such as a traffic accident etc.
The above object of the present invention can be achieved by a second method of detecting a vehicle speed pulse drop of a vehicle speed pulse in an on-vehicle navigation system, the system provided with a vehicle speed sensor, by which the vehicle speed pulse is generated, for a dead reckoning positioning, the method provided with: a measurement process of measuring a pulse interval between the vehicle speed pulses generated by the vehicle speed sensor, a first hold process of at least temporarily holding a pulse interval measured for the n-th time (n is a natural number) by the measurement process; and a detection process of detecting the vehicle speed pulse drop by using such a condition that a ratio of a pulse interval measured for the (n+m)-th time (m is a natural number) with respect to the pulse interval held by the first hold process is more than a predetermined threshold value as a detection condition.
According to the second method of the present invention, the measurement process measures the interval of the vehicle speed pulse. Then, the first hold process holds a pulse interval measured for the n-th time at least temporarily. For example, a buffer holds data, which show the pulse interval, every measurement. Then, the detection process detects the vehicle speed pulse drop by using such a condition that a ratio of a pulse interval measured for the (n+m)-th time with respect to the held pulse interval is more than a predetermined threshold value. For example, when the pulse interval held by the first hold process is regarded as W(n) and the pulse interval measured for the (n+m)-th time as W(n+1) and the threshold value as Wth, it is judged whether or not W (n+1)/W(n) greater than Wth every measurement or every time the vehicle speed pulse is generated. When this equation holds true, it is judged as the vehicle speed pulse drop. Therefore, as compared with the above mentioned first method of detecting the vehicle speed pulse drop, it is possible to detect the vehicle speed pulse drop by using the vehicle speed pulse exclusively without any radio wave positioning data such as GPS measurement data or the like. Thus, there is no use of the velocity averaged over a predetermined time duration, for example, {fraction (1/10)}, 1, or 2 sec, as in the case of the GPS measurement. Consequently, the detection accuracy in the detection of the vehicle speed pulse drop can be certainly more improved than the one of the first method in a situation that the accuracy of the GPS measurement is low, for example. It is also possible to detect the vehicle speed pulse drop with a high reliability in correspondence to such a generally low possibility that the vehicle speed pulse drop is continued to generate over a long period as long as a generation source of the vehicle speed pulse and the vehicle speed pulse operate properly.
Incidentally, in the second method, the detection process may detect the vehicle speed pulse drop by comparing a difference xcex94W between the pulse interval W(n) held by the first hold process and the pulse interval W(n+1) measured for the (n+m)-th time with another threshold value Wthxe2x80x2 (i.e. by using xcex94W=W(n+1)xe2x88x92W(n) greater than Wthxe2x80x2 as a detection condition).
In one aspect of the second method of the present invention, the method is further provided with a second hold process of at least temporarily holding the pulse interval measured for the (n+m)-th time, wherein the detection process uses such a condition that a ratio of the pulse interval held by the second hold process with respect to the pulse interval held by the first hold process is more than the predetermined threshold value as a detection condition.
According to this aspect, because such a condition that a ratio of the pulse interval held by the second hold process with respect to the pulse interval held by the first hold process is more than the predetermined threshold value is used as a detection condition, it is possible to detect the vehicle speed pulse drop relatively certainly.
In another aspect of the second method of the present invention, the predetermined threshold value is a variable value or a fixed value which is set in advance.
According to this aspect, as the predetermined threshold value, for example, the fixed value, which is set depending on a vehicle specification such as its weight, its engine ability, its brake control ability etc., may be used. Therefore, in the detection process, it is possible to detect the vehicle speed pulse drop by a simple comparison process such as a comparison of the fixed threshold value.
Moreover, as the predetermined threshold value, a variable value, which is variable depending on a velocity, may be used.
The above object of the present invention can be achieved by a third method of detecting a vehicle speed pulse drop of a vehicle speed pulse in an on-vehicle navigation system, the system provided with a vehicle speed sensor, by which the vehicle speed pulse is generated, for a dead reckoning positioning, the method provided with: a measurement process of measuring a pulse interval between the vehicle speed pulses generated by the vehicle speed sensor; a prediction process of predicting a generation time of the vehicle speed pulse, which is generated by the vehicle speed sensor and which defines an end of the pulse interval supposed to be measured for the (n+m)-th time (n and m are both natural numbers) by the measurement process, from the pulse interval measured for the n-th time by the measurement process; and a detection process of detecting the vehicle speed pulse drop, by using such a condition that the vehicle speed pulse is not generated by the vehicle speed pulse sensor even after it has elapsed the predicted generation time.
According to the third method of the present invention, the measurement process measures the pulse interval between vehicle speed pulses generated by the vehicle speed sensor. The prediction process predicts (calculates or estimates by calculating) the generation time of the vehicle speed pulse, which defines an end of the vehicle speed pulse interval to be measured for the (n+m)-th time, from the pulse interval measured for the n-th time. For example, when the pulse interval measured for the n-th time is regarded as W(n) and the pulse interval to be measured for the (n+m)-th time as W(n+1), in consideration of acceleration and deceleration of a vehicle, the W(n+1) is predicted by an expression such as W(n)xe2x88x92xcex94w less than W(n+1) less than W(n)+xcex94w or 1xe2x88x92xcex1 less than W(n+1)/W(n) less than 1+xcex1 (wherein, xcex94w and xcex1 are coefficients set in advance in consideration of acceleration and deceleration of a vehicle). Then, a generation time t1 of the vehicle speed pulse which defines an end of the predicted pulse interval W(n+1) is predicted (calculated) from a generation time t0 of the vehicle speed pulse generated for the last time (i.e., the vehicle pulse which defines an end of the pulse interval measured for the n-th time) and the calculated pulse interval W(n+1). Then, the detection process detects the vehicle speed pulse by such a condition that the vehicle speed pulse is not generated by the vehicle speed pulse sensor even after it has elapsed the predicted generation time t1, after comparing the current time with the predicted generation time t1 while monitoring a timer, for example. That is how to detect the vehicle speed pulse drop by using the vehicle speed pulse exclusively without any radio wave positioning data such as the GPS measurement data, as compared with the above mentioned the first method. Thus, there is no use of the velocity averaged over a predetermined time duration, for example, {fraction (1/10)}, 1, or 2 sec, as in the case of the GPS measurement. Consequently, the detection accuracy in the detection of the vehicle speed pulse drop can be certainly more improved than the one of the first method in a situation that the accuracy of the GPS measurement is low, for example. It is also possible to detect the vehicle speed pulse drop with a high reliability in correspondence to such a generally low possibility that the vehicle speed pulse drop is continued to generate over a long period as long as a generation source of the vehicle speed pulse. and the vehicle speed pulse operate properly.
In this case, the prediction process preferably predicts (calculates) the generation time with a certain width (period), and such a condition is preferably regarded as a detection condition that the vehicle speed pulse is not generated before the end point of the generation time (period).
In one aspect of the third method of the present invention, the method is further provided with a first hold process of at least temporarily holding the pulse interval measured for the n-th time, wherein the prediction process predicts the generation time from the pulse interval held by the first hold process.
According to this aspect, the generation time of the vehicle speed pulse which defines an end of the pulse interval to be measured for the (n+m)-th time is predicted (calculated) from the pulse interval measured for the n-th time which is held in a buffer or the like in the first hold process, so that it is possible to calculate the generation time relatively certainly and to detect the vehicle speed pulse on the basis of this prediction result.
In the prediction process, it is also possible to predict (calculate) the generation time of the vehicle speed pulse which defines an end of the (n+m)-th pulse interval from the pulse interval measured for the (nxe2x88x921)-th time in addition to the pulse interval measured for the n-th time.
In another aspect of the third method, the prediction process predicts the (n+m)-th pulse interval instead of predicting the generation time, and the detection process uses such a condition that a pulse interval shorter than the predicted pulse interval is not measured by the measurement process as the detection condition instead of using the condition that the vehicle speed pulse is not generated even after it has elapsed the predicted generation time.
According to this aspect, the prediction process predicts (calculates) the (n+m)-th pulse interval. Then the detection process detects the vehicle speed pulse drop by using such a condition that a pulse interval shorter than the predicted (calculated) pulse interval is not measured by the measurement process. In this manner, it is also possible to detect the vehicle speed pulse drop in the substantially same way by comparing the pulse interval measured in the measurement process with the predicted (calculated) pulse interval.
In another aspect of the first method, the detection process detects an excess generation of the vehicle speed pulse by the vehicle speed sensor by using the contradiction as a detection condition.
According to this aspect, xe2x80x9cthe excess generation of the vehicle speed pulsexe2x80x9d, which is phenomenon of imbalance between rotational frequency of a tire and a moving distance of a vehicle against road surface caused by racing or idling tire against the road surface depending on engine revolution and road surface condition etc. even in generating the vehicle speed pulse normally with respect to the rotation of a tire and a vehicle shaft, can be detected according to the similar theory as that of detection of the vehicle speed pulse in the first method. In other words, the excess generation of the vehicle speed pulse can be detected when the velocity based on the vehicle speed pulse increases so extremely that it cannot be accelerated in a real world on the basis of vehicle weight, engine output or the like as compared with the velocity based on the radio wave positioning data. Therefore, it is possible to improve an accuracy of the moving distance and the velocity based on the dead reckoning positioning measurement by correcting the vehicle speed pulse including an error caused by the excess generation when the excess generation of the vehicle speed pulse is detected. Alternatively, by temporarily stopping a use of the vehicle speed pulse in the excess generation and instead, by using the radio wave positioning data exclusively, the accuracy of the moving distance and the velocity can be improved.
In another aspect of the second method, the detection process detects the excess generation of the vehicle speed pulse by the vehicle speed sensor by using such a condition that a ratio of the pulse interval measured for the (n+m)-th time by the measurement process with respect to the pulse interval held by the first hold process is less than another threshold value as a detection condition.
According to this aspect, the excess generation of the vehicle speed pulse can be detected according to the similar theory as that of detection of the vehicle speed pulse drop in the second method. Namely, the excess generation of the vehicle speed pulse can be detected when the velocity based on the vehicle speed pulse increases so rapidly that it cannot be accelerated in a real world on the basis of vehicle weight, engine output or the like.
In another aspect of the third method, the detection process detects the excess generation of the vehicle speed pulse by the vehicle speed sensor by using such a condition that the vehicle speed pulse is generated by the vehicle speed sensor before the predicted generation time as a detection condition.
According to this aspect, the excess generation of the vehicle speed pulse can be detected according to the similar theory as that of detection of the vehicle speed pulse drop in the third method. Namely, the excess generation of the vehicle speed pulse can be detected when the velocity based on the vehicle speed pulse increases so rapidly that it cannot be accelerated in a real world on the basis of vehicle weight, engine output or the like.
In this case, the prediction process may preferably predict (calculate) the generation time with a certain width (period) and such a condition may be regarded as a detection condition that the vehicle speed pulse is not generated before an initiation point of the predicted generation time (period).
The above object of the present invention can be achieved by a first apparatus for detecting a vehicle speed pulse drop of a vehicle speed pulse in an on-vehicle navigation system, the system provided with a radio wave positioning apparatus and a vehicle speed sensor, by which the vehicle speed pulse is generated, for a dead reckoning positioning, the apparatus provided with: a first calculation device for calculating a first physical quantity of a predetermined type from radio wave positioning data of the radio wave positioning apparatus; a second calculation device for calculating a second physical quantity of the predetermined type from the vehicle speed pulse; and a detection device for detecting the vehicle speed pulse drop by using a contradiction caused between the calculated first physical quantity and the calculated second physical quantity as a detection condition.
According to the first apparatus of the present invention, the first calculation device calculates the first physical quantity of a predetermined type such as a velocity of a vehicle by using, for example, a frequency change caused by Doppler effect from the radio wave positioning data of the radio wave positioning apparatus such as a GPS receiver or the like. Along with this, the second calculation device calculates the second physical quantity of the same type as what is calculated in the first calculation device, e.g. a velocity, which is in proportion to the number of the vehicle speed pulse per unit time, of a vehicle, in which the on-vehicle navigation system is equipped, from the vehicle speed pulse such as the one from a vehicle speed sensor or the like. Then, the detection device detects the vehicle speed pulse drop by using contradiction caused between the calculated first physical quantity and the calculated second physical quantity, for example, such a contradiction that there is an enormous difference between the calculated two kinds of velocities of the vehicle, as a detection condition. Therefore, by using the radio wave positioning data with such a positioning accuracy that generally has no causal relationship with how easily the vehicle speed pulse drop occurs, it is possible to detect the vehicle speed pulse drop with a relatively high reliability. Thus, when the vehicle speed pulse drop occurs, it becomes possible to correct the vehicle speed pulse including an error of the vehicle speed sensor, or to eliminate a use of the vehicle speed pulse including the error for displaying a current position with the on-vehicle navigation system.
In one aspect of the first apparatus of the present invention, the first calculation device calculates a velocity as the first physical quantity, the second calculation device calculates a velocity as the second physical quantity, and the detection device uses such a condition that a difference between the velocity calculated by the first calculation device and the velocity calculated by the second calculation device is more than a predetermined threshold value as one detection condition for the vehicle speed pulse drop.
According to this aspect, the first calculation device calculates the velocity as the first physical quantity based on a positioning radio wave, and the second calculation device calculates the velocity as the second physical quantity based on the vehicle speed pulse, and the detection device detects the vehicle speed pulse drop by using such a condition that a difference between these velocities is more than the predetermined threshold value. Therefore, it is possible to detect the vehicle speed pulse drop with a high reliability by a relatively simple process of monitoring inequality between the velocity difference and the threshold value.
In this aspect of the first apparatus, the apparatus is further provided with a third calculation device for calculating a velocity change amount from the radio wave positioning data and a fourth calculation device for calculating a velocity change amount from the vehicle speed pulse, wherein the detection device uses such a condition that the velocity change amount calculated by the third calculation device is less than a predetermined threshold value and the velocity change amount calculated by the fourth calculation device is more than a predetermined threshold value as another detection condition for the vehicle speed pulse drop.
By constituting in this manner, the vehicle speed pulse drop is detected by using such a condition that the velocity change amount calculated by the third calculation device is less than the predetermined threshold value and the velocity change amount calculated by the fourth calculation device is more than the predetermined threshold value as a detection condition in addition to the condition that the above mentioned velocity difference is more than the predetermined threshold value. In this manner, by using the fact that the velocity change of a vehicle is to some degree within a limited range, it is possible to detect the vehicle speed pulse drop with a very high accuracy, except extremely special cases such as traffic accident etc.
The above object of the present invention can be achieved by a second apparatus for detecting a vehicle speed pulse drop of a vehicle speed pulse in an on-vehicle navigation system, the system provided with a vehicle speed sensor, by which the vehicle speed pulse is generated, for a dead reckoning positioning, the apparatus provided with: a measurement device for measuring a pulse interval between the vehicle speed pulses generated by the vehicle speed sensor, a first hold device for at least temporarily holding a pulse interval measured for the n-th time (n is a natural number) by the measurement device; and a detection device for detecting the vehicle speed pulse drop by using such a condition that a ratio of a pulse interval measured for the (n+m)-th time (m is a natural number) with respect to the pulse interval held by the first hold device is more than a predetermined threshold value as a detection condition.
According to the second apparatus of the present invention, the measurement device measures the interval of the vehicle speed pulse. Then, the first hold device holds a pulse interval measured for the n-th time at least temporarily. For example, a buffer holds data, which show the pulse interval, every measurement. Then, the detection device detects the vehicle speed pulse drop by using such a condition that a ratio of a pulse interval measured for the (n+m)-th time with respect to the held pulse interval is more than a predetermined threshold value. Therefore, as compared with the above mentioned first apparatus for detecting the vehicle speed pulse drop, it is possible to detect the vehicle speed pulse drop by using the vehicle speed pulse exclusively without any radio wave positioning data such as GPS measurement data or the like.
Incidentally, in the second apparatus, the detection process may detect the vehicle speed pulse drop by comparing a difference between the pulse interval held by the first hold device and the pulse interval measured for the (n+m)-th time with another threshold value.
In one aspect of the second apparatus of the present invention, the apparatus is further provided with a second hold device of at least temporarily holding the pulse interval measured for the (n+m)-th time, wherein the detection device uses such a condition that a ratio of the pulse interval held by the second hold device with respect to the pulse interval held by the first hold device is more than the predetermined threshold value as a detection condition.
According to this aspect, because such a condition that a ratio of the pulse interval held by the second hold device with respect to the pulse interval held by the first hold device is more than the predetermined threshold value is used as a detection condition, it is possible to detect the vehicle speed pulse drop relatively certainly.
In another aspect of the second apparatus of the present invention, the predetermined threshold value is a variable value or a fixed value which is set in advance.
According to this aspect, as the predetermined threshold value, for example, the fixed value, which is set depending on a vehicle specification such as its weight, its engine ability, its brake control ability etc., may be used. Therefore, in the detection device, it is possible to detect the vehicle speed pulse drop by a simple comparison device such as a comparison of the fixed threshold value.
Moreover, as the predetermined threshold value, a variable value, which is variable depending on a velocity, may be used.
The above object of the present invention can be achieved by a third apparatus for detecting a vehicle speed pulse drop of a vehicle speed pulse in an on-vehicle navigation system, the system provided with a vehicle speed sensor, by which the vehicle speed pulse is generated, for a dead reckoning positioning, the apparatus provided with: a measurement device for measuring a pulse interval between the vehicle speed pulses generated by the vehicle speed sensor; a prediction device for predicting a generation time of the vehicle speed pulse, which is generated by the vehicle speed sensor and which defines an end of the pulse interval supposed to be measured for the (n+m)-th time (n and m are both natural numbers) by the measurement device, from the pulse interval measured for the n-th time by the measurement device; and a detection device for detecting the vehicle speed pulse drop, by using such a condition that the vehicle speed pulse is not generated by the vehicle speed pulse sensor even after it has elapsed the predicted generation time.
According to the third method of the present invention, the measurement process measures the pulse interval between vehicle speed pulses generated by the vehicle speed sensor. The calculation process calculates or predicts the generation time of the vehicle speed pulse, which defines an end of the vehicle speed pulse interval to be measured for the (n+m)-th time, from the pulse interval measured for the n-th time. Then, a generation time of the vehicle speed pulse which defines an end of the calculated pulse interval is predicted (calculated) from a generation time of the vehicle speed pulse generated for the last time and the predicted (calculated) pulse interval. Then, the detection process detects the vehicle speed pulse by such a condition that the vehicle speed pulse is not generated by the vehicle speed pulse sensor even after it has elapsed the predicted generation time, after comparing the current time with the predicted (calculated) generation time while monitoring a timer, for example. That is how to detect the vehicle speed pulse drop by using the vehicle speed pulse exclusively without any radio wave positioning data such as the GPS measurement data, as compared with the above mentioned the first method.
In this case, the prediction device may preferably predict (calculate) the generation time with a certain width (period), and such a condition may be preferably regarded as a detection condition that the vehicle speed pulse is not generated before the end point of the predicted generation time (period).
In one aspect of the third apparatus of the present invention, the apparatus is further provided with a first hold device of at least temporarily holding the pulse interval measured for the n-th time, wherein the prediction device predicts (calculates) the generation time from the pulse interval held by the first hold device.
According to this aspect, the generation time of the vehicle speed pulse which defines an end of the pulse interval to be measured for the (n+m)-th time is predicted (calculated) from the pulse interval measured for the n-th time which is held in a buffer or the like in the first hold device, so that it is possible to predict (calculate) the generation time relatively certainly and to detect the vehicle speed pulse on the basis of this prediction result.
In another aspect of the third apparatus, the prediction device predicts the (n+m)-th pulse interval instead of predicting the generation time, and the detection device uses such a condition that a pulse interval shorter than the predicted pulse interval is not measured by the measurement device as the detection condition instead of using the condition that the vehicle speed pulse is not generated even after it has elapsed the predicted generation time.
According to this aspect, the prediction device predicts (calculates) the (n+m)-th pulse interval. Then the detection device detects the vehicle speed pulse drop by using such a condition that a pulse interval shorter than the predicted (calculated) pulse interval is not measured by the measurement device. In this manner, it is also possible to detect the vehicle speed pulse drop in the substantially same way by comparing the pulse interval measured in the measurement device with the predicted (calculated) pulse interval.
In another aspect of the first apparatus, the detection device detects an excess generation of the vehicle speed pulse by the vehicle speed sensor by using the contradiction as a detection condition.
According to this aspect, the excess generation of the vehicle speed pulse can be detected according to the similar theory as that of detection of the vehicle speed pulse in the first apparatus. In other words, the excess generation of the vehicle speed pulse can be detected when the velocity based on the vehicle speed pulse increases so extremely that it cannot be accelerated in a real world on the basis of vehicle weight, engine output or the like as compared with the velocity based on the radio wave positioning data.
In another aspect of the second apparatus, the detection device detects the excess generation of the vehicle speed pulse by the vehicle speed sensor by using such a condition that a ratio of the pulse interval measured for the (n+m)-th time by the measurement device with respect to the pulse interval held by the first hold device is less than another threshold value as a detection condition.
According to this aspect, the excess generation of the vehicle speed pulse can be detected according to the similar theory as that of detection of the vehicle speed pulse drop in the second apparatus. Namely, the excess generation of the vehicle speed pulse can be detected when the velocity based on the vehicle speed pulse increases so rapidly that it cannot be accelerated in a real world on the basis of vehicle weight, engine output or the like.
In another aspect of the third apparatus, the detection device detects the excess generation of the vehicle speed pulse by the vehicle speed sensor by using such a condition that the vehicle speed pulse is generated by the vehicle speed sensor before the predicted generation time as a detection condition.
According to this aspect, the excess generation of the vehicle speed pulse can be detected according to the similar theory as that of detection of the vehicle speed pulse drop in the third apparatus. Namely, the excess generation of the vehicle speed pulse can be detected when the velocity based on the vehicle speed pulse increases so rapidly that it cannot be accelerated in a real world on the basis of vehicle weight, engine output or the like.
In this case, the prediction device may preferably predict (calculate) the generation time with a certain width (period) and such a condition is regarded as a detection condition that the vehicle speed pulse is not generated before an initiation point of the predicted generation time (period).
The above object of the present invention can be achieved by an on-vehicle navigation system provided with: any one of the above-mentioned first to third apparatuses for detecting a vehicle speed pulse drop of the present invention (including its various aspects); the vehicle speed sensor; a correction device for applying a correction onto the vehicle speed pulse when the vehicle speed pulse drop is detected by the apparatus for detecting the vehicle speed pulse drop; and a display device for displaying a current position based on the vehicle speed pulse.
According to this aspect of the on-vehicle navigation system of the present invention, because it is provided with any one of the first to third apparatuses for detecting the vehicle speed pulse drop of the present invention as described above, even if the vehicle speed pulse drop occurs, it can be detected with a high accuracy. Therefore, the correction device can corrects the vehicle speed pulse including an error caused by the vehicle speed pulse drop pulse, so that the display device can display a current position based on the vehicle speed pulse with a high accuracy.
In one aspect of the on-vehicle navigation system of the present invention, the system is further provided with a radio wave positioning apparatus for outputting radio wave positioning data, wherein the display device displays the current position based on the radio wave positioning data in addition to the vehicle speed pulse.
According to this aspect, the display device displays the current position based on the radio wave positioning data in addition to the vehicle speed pulse, and thus, even if the vehicle speed pulse drop occurs, it can be detected with a high accuracy and it may become possible not to use the vehicle speed pulse including an error caused by the vehicle speed pulse drop for calculation of the current position. Therefore, the current position can be displayed even if the vehicle speed pulse drop occurs.
The above object of the present invention can be also achieved by a program storage device readable by a computer. The program storage device stores a program of instructions to cause the computer to function as at least one portion of any one of the above described first to third methods of the present invention (including its various aspects).
According to the program storage device, such as a CD-ROM (Compact Discxe2x80x94Read Only Memory), a ROM, a DVD-ROM (DVD Read Only Memory), a floppy disk or the like, of the present invention, the above described first method of the present invention can be relatively easily realized as a computer reads and executes the program of instructions or as it executes the program after downloading the program through communication device. Moreover, the program of instructions can be sent from a central device with an application program required for the navigation or other data such as a map.
The above object of the present invention can be also achieved by a first computer data signal embodied in a carrier wave and representing a series of instructions for a computer. The series of instructions causes the computer to function as at least one portion of any one of the above described first to third methods of the present invention (including its various aspects).
According to the computer data signal embodied in the carrier wave of the present invention, as the computer downloads the program in the computer data signal through a computer network or the like, and executes this program, it is possible to realize the above described first method of the present invention.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with reference to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.